1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a method for forming a transparent electrode and a semiconductor device manufactured using the method.
2. Description of the Related Art
Transparent electrodes have been used in various application fields such as LEDs, solar cells, medical UV sterilizers, and fisheries, and the application fields and their demands have been gradually increased. Particularly, the transparent electrodes have been actively used in the LED field. The transparent electrode technique currently applied to the LEDs is mainly an ITO (Indium Tin Oxide) based technique which can be applied to a visible wavelength range of 400 nm to 800 nm and a UV wavelength range of 365 nm to 400 nm in the entire UV wavelength range of 10 nm to 400 nm.
Recently, demands for UV LEDs generating light in a UV wavelength range has been greatly increased. However, a transparent electrode having high conductivity and high transmittance in the UV wavelength range has not been developed, so that it is difficult to commercialize the UV LEDs.
For example, in the case of a UV LED where an ITO transparent electrode which is currently actively used is formed, most of light in a UV wavelength range of 10 nm to 320 nm generated in an activation layer is absorbed by an ITO layer, so that only about 1% of the light can be transmitted through the ITO layer to be extracted to an external portion.
FIG. 1 is a graph illustrating transmittance in the case where an ITO transparent electrode is formed on a p-GaN semiconductor layer in the related art. As illustrated in FIG. 1, although the transparent electrode has transmittance of 80% or more with respect to the light in a wavelength range of 350 nm or more, the transmittance is greatly decreased with respect to the light having a short wavelength in the UV wavelength range. Particularly, the transmittance is decreased to 20% or less with respect to the light having a short wavelength in the UV wavelength range of 280 nm or less.
In order to solve the above problem, in the related art, a metal electrode pad is directly formed on a semiconductor layer such as p-AlGaN instead of forming the transparent electrode on the semiconductor layer. However, the metal and the semiconductor layer are not in ohmic contact to each other because of a large difference in work function between the metal and the semiconductor layer, and current is concentrated on a metal electrode pad, but current is not supplied into the entire activation layer, so that an amount of the light generated by the activation layer is greatly decreased.
In order to solve the above problem, various researches have been made, but a transparent electrode having high conductivity and high transmittance in a UV wavelength range has not yet been developed. This is because conductivity and transmittance of a material is basically in trade-off relationship. Since a material having high transmittance in a UV wavelength range has a large band gap, the conductivity thereof is too low to be used as an electrode, and since the material is not in ohmic contact with a semiconductor material, it is impossible to use the above material as an electrode.
As an example of a technique for solving the above problem, a technique where a transparent electrode is constructed with a sliver (Ag) thin film is disclosed in Korean Patent Application No. 10-2007-0097545. However, in the related art, in the case where the transparent electrode is formed by using Ag, it is very difficult to deposit a thin silver layer on a semiconductor layer so that the thin sliver layer is in ohmic contact with the semiconductor layer. In addition, although a thin silver layer is deposited on the semiconductor layer, as illustrated in the graph of FIG. 4 of the above Patent Document, with respect to the light in a wavelength range 420 nm or less, the transmittance is greatly decreased to 80% or less; and with respect to the light in a wavelength range 380 nm or less, the transmittance is decreased to 50% or less. Therefore, the transmittance in the above-described technique has no difference from the transmittance of the ITO transparent electrode in the related art, and thus, it is difficult to improve the transmittance in a UV wavelength range up to a practical level.